<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<script type="text/javascript">
    function onchangeFun() {
        alert("英雄已选择");

    }
    window.onload = function () {
        var id = document.getElementById("01");
        id.onchange =function () {
            alert("英雄已选择");

        }

    }


</script>
<body>

<select onchange="onchangeFun()">
    <option value="">请选择你的英雄</option>
    <option value="">末日铁拳</option>
    <option value="">源氏</option>
    <option value="">黑百合</option>
    <option value="">半藏</option>
</select>

<select id="01">
    <option value="">请选择你的英雄</option>
    <option value="">麦克雷</option>
    <option value="">猎空</option>
    <option value="">艾什</option>
    <option value="">路霸</option>
</select>

</body>
</html>